Regarding techniques for improving the mechanical properties of cold-rolled steel sheets, following Patent Document 1 indicated discloses a high-strength steel sheet having bainitic ferrite as a main phase, and containing at least 3% lath type austenite and 1% to (lath type retained austenite area occupancy ratio×½) block-type austenite, the steel sheet being described as having excellent ductility and stretch flangeability. However, the block-type austenite in this steel sheet has grain diameters of around 2.2 μm to 20 μm and thus is coarse, and therefore can be considered as adversely affecting the formability of the steel sheet.
Patent Document 2 discloses a method for performing cold rolling using a hot-rolled steel sheet manufactured by hot rolling and then starting cooling in a short period of time after the hot rolling. For example, Patent Document 2 discloses that a hot-rolled steel sheet having a fine structure containing ferrite having a small average grain diameter as a main phase is manufactured by performing cooling to at most 720° C. at a cooling rate of at least 400° C./sec within 0.4 seconds after hot rolling and the hot-rolled steel sheet is subjected to usual cold rolling and annealing to manufacture a cold-rolled steel sheet. Hereinafter, the above hot-rolled steel sheet production process may also be referred to as immediate cooling method.